The present invention relates to a clock-generating circuit and image-forming apparatus employing a polygon mirror in its optical writing section, and specifically relates to a combination of an image-forming apparatus and a clock-generating circuit, which makes it possible to reduce influence to the image quality caused by variations of transmittances and reflectances in the optical-writing section when the polygon mirror scans a laser beam.
In conventional image-forming apparatus, an image is formed on an image-bearing member rotating in a sub-scanning direction by scanning a laser beam, modulated in response to image data, in a main-scanning direction. In this operation, the laser beam is modulated in response to the image data in reference to reference signals called as dot clock pulses.
Accordingly, in accordance with a predetermined number of dot clock pulses, it is necessary to generate the dot clock pulses in order that the length of the image, formed on the image-bearing member, is always kept constant in the main-scanning direction.
Further, in recent years it has been developed a color image forming apparatus equipped with a plurality of units comprising means for charging, exposure, and development respectively in the vicinity of the image-bearing member, on which a full color toner image is formed during one revolution of it, and transferring the full color toner image onto a recording paper at a time. On the other hand, it has been also developed a color image forming apparatus equipped with a plurality of image-bearing members in the vicinity of an intermediate transfer member and equipped with means for charging, exposure, development and transfer around each of the image-bearing members respectively, transferring the toner images having been formed on the respective image-bearing members sequentially onto the intermediate transfer member, and further transferring the full color toner image born on the intermediate transfer member onto the recording paper at a time.
Incidentally, FIG. 8 shows perspective view of a configuration of optical writing section 490 employing a laser beam in the image-forming apparatus as described in the above. As shown in FIG. 8, laser diode 470 (LD) emits the laser beam based on the laser driving signals generated by circuit section 400.
Further, the laser beam, emitted by laser diode 470 and having been passed through collimator lens 491 and cylindrical lens 492, is deflected for scanning by polygon mirror 493, and scans image-bearing member 1 for writing, after passing through fxcex8 lens 494 so as to adjust the scanning velocity at constant and cylindrical lens 495. In addition, a part of the laser beam deflected by polygon mirror 493 is led to index sensor 412 for detecting the timing.
[First Problem]
As shown in FIG. 9, the laser beam, deflected for scanning by polygon mirror 493, is led to image-bearing member 1 after passing through the optical elements, including fxcex8 lens 494, cylindrical lens 465 and dustproof filter 496.
During the abovementioned scanning operation, since the laser beam enters into each of the optical elements from a direction substantially orthogonal to them at a low incident angle when the laser beam scans in the vicinity of the scanning center, the reflectances become low values while the transmittances become high values. On the contrary, since the laser beam enters into each of the optical elements from a direction inclined to them at a high incident angle when the laser beam scans in both end portions of the scanning region, the reflectances become high values while the transmittances become low values. Incidentally, the length of the arrow in FIG. 9 illustratively indicates the intensity of the reflected light.
The variation of the light intensity to be irradiated onto image-bearing member 1, caused by the above-mentioned changes of reflectances and transmittances, results in a quality deterioration of the image formed on image-bearing member 1, which is called as an unevenness of scanning-light amount (a shading). Although the scanning-light amount is generally apt to decrease at the both end portions of the scanning region, the shading property would vary depending on the shapes and arrangements of various kinds of lenses and filters included in the optical system, as shown in FIG. 10.
Further, in a certain manner of disposing dustproof filter 496, the scanning-light amount would be possibly lowered at only one end portion of the main-scanning region. In the example shown in FIG. 9, since an amount of light reflected by the dustproof filter 496 increases at the right end portion of the main-scanning region, the unevenness of the scanning-light amount would be apt to also increase. Accordingly, as for conventional image-forming apparatus, not only the mechanically optimum disposition, but also the unevenness of the scanning-light amount has restricted the design flexibility of disposing dustproof filter 496.
Still further, to suppress the above-mentioned unevenness of the scanning-light amount as lower as possible, it has been effective to apply an expensive multi-coating treatment onto each of the optical elements concerned. It has been a problem, however, that such the treatment surely raises the total cost of the optical elements.
[Second Problem]
With respect to the first problem mentioned above, in an image-forming apparatus in which multi-valued pulses for driving the laser diode are generated in the PWM (Pulse Width Modulation) process, it would be an applicable countermeasure that the PWM values, generated at a portion where the scanning-light amount does not decrease, are lowered to a level equivalent to the light amount to be reduced by the shading effect, so as to cancel the unevenness of the image caused by the shading effect. For instance, when the scanning-light amount decreases by 15% of it at the end portion of the main-scanning region due to the shading effect, it is possible to keep the resulted scanning-light amount constant over the whole region of the image by lowering the PWM values for the portion where the scanning-light amount does not decrease.
The abovementioned countermeasure, however, restricts the dynamic-range of the scanning-light intensity at a maximum light amount side. In the above example, the dynamic-range of the scanning-light intensity is restricted within a range of 0-85%. In other words, although the abovementioned countermeasure can suppress the shading effect, there would arise another problem that such the countermeasure adversely affects the gradation characteristic of the image.
To overcome the abovementioned drawbacks in conventional image-forming apparatus, it is an object of the present invention to provide a clock-generating circuit and an image-forming apparatus employing the polygon mirror in its optical writing system, which make it possible to reduce the influence to the image quality, caused by the unevenness of the scanning-light amount (the shading effect).
Accordingly, to overcome the cited shortcomings, the abovementioned object of the present invention can be attained by clock-generating circuits and image-forming apparatus described as follow.
(1) A circuit for generating dot clock pulses for driving a light-emitting element employed in an optical-writing section of an image-forming apparatus, comprising: a digital-delay dot clock adjusting section to adjust timings of rising-edges or falling-edges of the dot clock pulses generated by changing a selection for a plurality of delayed-clock pulses, which are generated by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times; and a controlling section to control a selecting operation for the plurality of delayed clock pulses, performed in the digital-delay dot clock adjusting section, so as to compensate for unevenness of scanning-light amount caused by an optical element employed in the optical-writing section.
(2) A circuit for generating dot clock pulses for driving a light-emitting element employed in an optical-writing section of an image-forming apparatus, comprising: an index sensor to detect a light-beam, which is emitted from the light-emitting element and is deflected for scanning by a light-scanning device employed in the optical-writing section, and to output an index signal when the index sensor detects the light-beam at an end portion of a main-scanning region scanned by the light-beam; a delay-chain section to generate a plurality of delayed-clock pulses by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times; a synchronized clock pulse detecting section to select synchronized delayed-clock pulses, which are synchronized with the index signal, out of the plurality of delayed clock pulses generated in the delay-chain section, and to output a number of delay-stages, which is derived from the synchronized delayed-clock pulses, as synchronizing information; a table memory to store scanning-light unevenness information, which represents unevenness of scanning-light amounts caused by an optical element employed in the optical-writing section; a delayed-clock switching section to generate a select signal, for selecting a specific delayed-clock pulse, having a phase suitable for compensating for the unevenness of scanning-light amount caused by the optical element employed in the optical-writing section, out of the plurality of delayed-clock pulses, based on the synchronized delayed-clock pulses and the synchronizing information outputted from the synchronized clock pulse detecting section and the scanning-light unevenness information stored in the table memory; and a selector to select the specific delayed-clock pulse out of the plurality of delayed-clock pulses in response to the select signal generated by the delayed-clock switching section.
(3) The circuit of item 2, wherein the select signal is generated in the delayed-clock switching section by performing a calculating operation, based on the synchronized delayed-clock pulses outputted by the synchronized clock pulse detecting section and the scanning-light unevenness information stored in the table memory.
(4) The circuit of item 2, wherein the select signal is generated in the delayed-clock switching section by performing a table-converting operation, based on the synchronized delayed-clock pulses outputted by synchronized clock pulse detecting section and the scanning-light unevenness information stored in the table memory.
(5) The circuit of item 2, wherein the unevenness of scanning-light amount is a variation of light amount due to non-uniformity of reflectance and/or transmittance caused by the optical element.
(6) The circuit of item 2, wherein the delay-chain section, the synchronized clock pulse detecting section, the table memory and the selector are digital circuits fabricated in an integrated circuit.
(7) An image-forming apparatus, comprising: an image-forming section that includes an optical-writing section to form an image; and a circuit for generating dot clock pulses for driving a light-emitting element employed in the optical-writing section included in the image-forming section;
wherein the circuit includes, a digital-delay dot clock adjusting section to adjust timings of rising-edges or falling-edges of the dot clock pulses generated by changing a selection for a plurality of delayed-clock pulses, which are generated by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times, and a controlling section to control a selecting operation for the plurality of delayed clock pulses, performed in the digital-delay dot clock adjusting section, so as to compensate for unevenness of scanning-light amount caused by an optical element employed in the optical-writing section;
and wherein the image-forming section performs an image-forming operation based on clock signals outputted from the digital-delay dot clock adjusting section.
(8) An image-forming apparatus, comprising: an image-forming section that includes an optical-writing section to form an image; and a circuit for generating dot clock pulses for driving a light-emitting element employed in the optical-writing section included in the image-forming section;
wherein the circuit includes: an index sensor to detect a light-beam, which is emitted from the light-emitting element and is deflected for scanning by a light-scanning device employed in the optical-writing section, and to output an index signal when the index sensor detects the light-beam at an end portion of a main-scanning region scanned by the light-beam; a delay-chain section to generate a plurality of delayed-clock pulses by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times; a synchronized clock pulse detecting section to select synchronized delayed-clock pulses, which are synchronized with the index signal, out of the plurality of delayed clock pulses generated in the delay-chain section, and to output a number of delay-stages, which is derived from the synchronized delayed-clock pulses, as synchronizing information; a table memory to store scanning-light unevenness information, which represents unevenness of scanning-light amounts caused by an optical element employed in the optical-writing section; a delayed-clock switching section to generate a select signal, for selecting a specific delayed-clock pulse, having a phase suitable for compensating for the unevenness of scanning-light amount caused by the optical element employed in the optical-writing section, out of the plurality of delayed-clock pulses, based on the synchronized delayed-clock pulses and the synchronizing information outputted from the synchronized clock pulse detecting section and the scanning-light unevenness information stored in the table memory; and a selector to select the specific delayed-clock pulse out of the plurality of delayed-clock pulses in response to the select signal generated by the delayed-clock switching section; and wherein the image-forming section performs an image-forming operation based on clock signals outputted from the selector.
(9) The image-forming apparatus of item 8, wherein the select signal is generated in the delayed-clock switching section by performing a calculating operation, based on the synchronized delayed-clock pulses outputted by the synchronized clock pulse detecting section and the scanning-light unevenness information stored in the table memory.
(10) The image-forming apparatus of item 8, wherein the select signal is generated in the delayed-clock switching section by performing a table-converting operation, based on the synchronized delayed-clock pulses outputted by synchronized clock pulse detecting section and the scanning-light unevenness information stored in the table memory.
(11) The image-forming apparatus of item 8, wherein the unevenness of scanning-light amount is a variation of light amount due to non-uniformity of reflectance and/or transmittance caused by the optical element.
(12) The image-forming apparatus of item 8, wherein the delay-chain section, the synchronized clock pulse detecting section, the table memory and the selector are digital circuits fabricated in an integrated circuit.
Further, to overcome the abovementioned problems, other clock-generating circuits and image-forming apparatus, embodied in the present invention, will be described as follow:
(13) A clock-generating circuit for generating dot clock pulses for driving a light-emitting element employed in a writing section of an image-forming apparatus, characterized by comprising:
digital-delay dot clock adjusting means for changing timings of rising-edges or falling-edges of dot clock pulses generated by changing a selection for a plurality of delayed-clock pulses, which are generated by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times; and
controlling means for controlling a selection of the plurality of delayed clock pulses, performed in the digital-delay dot clock adjusting means, so as to compensate for unevenness of scanning-light amount caused by an optical element employed in the writing section.
(14) An image-forming apparatus characterized by comprising:
digital-delay dot clock adjusting means for changing timings of rising-edges or falling-edges of dot clock pulses generated by changing a selection for a plurality of delayed-clock pulses, which are generated by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times;
controlling means for controlling a selection of the plurality of delayed clock pulses, performed in the digital-delay dot clock adjusting means, so as to compensate for unevenness of scanning-light amount caused by an optical element employed in a writing section; and
image-forming means for performing image-forming operations based on clock signals from the digital-delay dot clock adjusting means; and
characterized in that pulse-widths of dot clock pulses are changed by controlling selections of a plurality of delayed clock signals so as to compensate for the unevenness of scanning-light amount.
According to the above invention, since a plurality of delayed-clock pulses are generated by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times, and a selection of the plurality of delayed clock pulses is changed in response to the unevenness of scanning-light amount, it is possible to cancel the unevenness of scanning-light amount on the surface of the image-bearing member by slightly changing timings of rising-edges or falling-edges of dot clock pulses.
As a result, in the image-forming apparatus incorporating a polygon mirror in the optical writing system, and its clock generating circuit, it becomes possible to reduce the influence to the image quality, caused by the unevenness of scanning-light amount, with a simple circuit configuration.
(15) A clock-generating circuit generating dot clock pulses for driving a light-emitting element employed in a writing section of an image-forming apparatus, characterized by comprising:
a delay-chain section to generate a plurality of delayed-clock pulses by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times, a synchronized clock pulse detecting section to select
a plurality of delayed-clock pulses (synchronized delayed-clock pulses), which are synchronized with an index signal being a reference of an end portion, from the delay-chain section, and to output a number of delay-stages of the delay-chain section from the synchronized delayed-clock pulses, as synchronizing information;
a table to hold scanning-light unevenness information, pertaining to unevenness of scanning-light caused by an optical element used in the writing section;
a synchronized switching section to generate a select signal, for determining what phased delayed-clock pulse should be selected out of the plurality of delayed-clock pulses from the synchronized delayed-clock pulses and the synchronizing information outputted from the synchronized clock pulse detecting section and the scanning-light unevenness information from the table; and
a signal selecting section to select the delayed-clock pulse corresponding to the select signal out of the plurality of delayed-clock pulses; and
characterized in that pulse-widths of dot clock pulses are changed by controlling selections of a plurality of delayed clock signals so as to compensate for the unevenness of scanning-light amount.
(16) An image-forming apparatus characterized by comprising:
a delay-chain section to generate a plurality of delayed-clock pulses by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times,
a synchronized clock pulse detecting section to select a plurality of delayed-clock pulses (synchronized delayed-clock pulses), which are synchronized with an index signal being a reference of an end portion, from the delay-chain section, and to output a number of delay-stages of the delay-chain section from the synchronized delayed-clock pulses, as synchronizing information;
a table to hold scanning-light unevenness information, pertaining to unevenness of scanning-light caused by an optical element used in the writing section;
a synchronized switching section to generate a select signal, for determining what phased delayed-clock pulse should be selected out of the plurality of delayed-clock pulses from the synchronized delayed-clock pulses and the synchronizing information outputted from the synchronized clock pulse detecting section and the scanning-light unevenness information from the table;
a signal selecting section to select the delayed-clock pulse corresponding to the select signal out of the plurality of delayed-clock pulses; and
image-forming means for performing an image-forming operation based on the clock signals outputted from the signal selecting section; and
characterized in that pulse-widths of dot clock pulses are changed by controlling selections of a plurality of delayed clock signals so as to compensate for the unevenness of scanning-light amount.
According to the above invention, since the delay-chain section generates the plurality of delayed-clock pulses by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times, and the selection of the plurality of delayed-clock pulses in the signal selecting section is changed by the select signal from the synchronized clock pulse detecting section based on the synchronizing information from the synchronized clock pulse detecting section and the scanning-light unevenness information held in the table, it is possible to cancel the unevenness of scanning-light amount on the surface of the image-bearing member by slightly changing timings of rising-edges or falling-edges of dot clock pulses and thereby changing the pulse-widths of pulses for driving the light-emitting element.
As a result, in the image-forming apparatus incorporating a polygon mirror in the optical writing system, and its clock generating circuit, it becomes possible to reduce the influence to the image quality, caused by the unevenness of scanning-light amount, with a simple circuit configuration.
(17) The clock-generating circuit described in item 15, characterized in that synchronized switching section generates the select signal by performing a calculating operation, from the synchronized delayed-clock pulses from the synchronized clock pulse detecting section and the scanning-light unevenness information from the table.
(18) The image-forming apparatus described in item 16, characterized in that synchronized switching section generates the select signal by performing a calculating operation, from the synchronized delayed-clock pulses from the synchronized clock pulse detecting section and the scanning-light unevenness information from the table.
According to the above invention, since a plurality of delayed-clock pulses are generated by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times, and the selection of the plurality of delayed-clock pulses is changed in response to the result of calculations performed by referring to the unevenness of scanning-light amount, it is possible to cancel the unevenness of scanning-light amount on the surface of the image-bearing member by slightly changing timings of rising-edges or falling-edges of dot clock pulses and thereby changing the pulse-widths of pulses for driving the light-emitting element.
As a result, in the image-forming apparatus incorporating a polygon mirror in the optical writing system, and its clock generating circuit, it becomes possible to reduce the influence to the image quality, caused by the unevenness of scanning-light amount, with a simple circuit configuration.
(19) The clock-generating circuit described in item 15, characterized in that synchronized switching section generates the select signal by performing a table-converting operation, from the synchronized delayed-clock pulses from the synchronized clock pulse detecting section and the scanning-light unevenness information from the table.
(20) The image-forming apparatus described in item 16, characterized in that synchronized switching section generates the select signal by performing a table-converting operation, from the synchronized delayed-clock pulses from the synchronized clock pulse detecting section and the scanning-light unevenness information from the table.
According to the above invention, since a plurality of delayed-clock pulses are generated by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times, and the selection of the plurality of delayed-clock pulses is changed in response to the result of a table-converting operation performed by using a look-up table while referring to the unevenness of scanning-light amount, it is possible to cancel the unevenness of scanning-light amount on the surface of the image-bearing member by slightly changing timings of rising-edges or falling-edges of dot clock pulses and thereby changing the pulse-widths of pulses for driving the light-emitting element.
As a result, in the image-forming apparatus incorporating a polygon mirror in the optical writing system, and its clock generating circuit, it becomes possible to reduce the influence to the image quality, caused by the unevenness of scanning-light amount, with a simple circuit configuration.
(21) The clock-generating circuit described in any one of items 13, 15, 17 and 19, characterized in that the unevenness of scanning-light amount is a variation of light amount based on a difference of reflectance or transmittance of the optical element.
(22) The image-forming apparatus described in any one of items 14, 16, 18 and 20, characterized in that the unevenness of scanning-light amount is a variation of light amount based on difference of reflectance or transmittance of the optical element.
According to the above invention, in the image-forming apparatus incorporating the polygon mirror in the optical writing system, and its clock generating circuit, it becomes possible to reduce the influence to the image quality, caused by the unevenness of scanning-light amount (a variation of light amount due to non-uniformity of reflectance and/or transmittance caused by the optical element), with a simple circuit configuration.
(23) The clock-generating circuit described in any one of items 13, 15, 17, 19, and 21, characterized in that each of the sections is structured as a digital circuit in an integrated circuit.
(24) The image-forming apparatus described in any one of items 14, 16, 18, 20, and 22, characterized in that each of the sections is structured as a digital circuit in an integrated circuit.
According to the above invention, in the image-forming apparatus incorporating a polygon mirror in the optical writing system, and its clock generating circuit, it becomes possible to reduce the influence to the image quality, caused by the unevenness of scanning-light amount, with a simple configuration of digital circuits excluding any analogue circuit.